Aerial refueling of aircraft enables an aircraft to fly farther without landing to refuel, allows an aircraft to continue flight in situations in which no suitable landing fields are available, and allows an aircraft to carry more payload instead of fuel as this fuel can be replenished in flight. In refueling operations, the aircraft that carries and transfers the fuel is the “tanker”, and the aircraft that receives this fuel is the “receiver”.
The two principal conventional methods of aerial refueling are the “boom/receptacle” method and the “probe/drogue” method.
In the boom/receptacle method, the tanker is equipped with a rigid, pivoted and telescoping boom at the lower aft end of the belly of the aircraft. To connect the boom to the receptacle on the receiver aircraft, the receiver aircraft first flies into a position below and behind the tanker. A boom operator then lowers and aligns the boom with the receiver aircraft's receptacle before extending the boom into the receptacle to hook up the connection for fuel transfer.
In the probe/drogue method, the tanker is equipped with a flexible hose which has a drogue or basket at one end of the flexible hose. The flexible hose may be located on each wing of the tanker. The receiver aircraft, on the other hand, is equipped with a retractable or fixed rigid probe. To connect the drogue or basket to the probe, the tanker first trails the flexible hose with the drogue or basket. The receiver aircraft then lines up the probe with the drogue or basket before moving forward to plug the probe into the drogue or basket to hook up the connection for fuel transfer.
The boom/receptacle method's main disadvantage is that each tanker can only be equipped with one boom. This is due to the size and weight of the conventional boom, which requires the boom to be designed to occupy a position at the lower aft end of the belly of the tanker aircraft so that the tanker can safely carry the boom. In turn, this means that the tanker can only refuel one receiver aircraft at a time. This issue is amplified when refueling is required for multiple receiver aircraft.
For example, when multiple aircraft are requesting refueling from a tanker using the boom/receptacle method, only one receiver aircraft can be refueled at a time, while the remaining receiver aircraft must “queue up” and wait for their turn. If the first receiver aircraft requires five minutes to connect with and stay “on the boom” to top up its tanks, a flight of four receiver aircraft requires 20 minutes to top up their tanks; by which time the first receiver aircraft would have waited for 15 minutes, resulting in it having consumed 15 minutes worth of fuel, while the second receiver aircraft has consumed 10 minutes worth of fuel, and so on. This problem is exacerbated when there is a high receiver-to-tanker ratio, e.g. eight receiver aircraft per tanker.
For example, in military combat aerial refueling operations, this may result in the fighters having different levels of fuel when the entire flight of fighters completes the refueling. For one fighter in a flight to have significantly lower fuel than the other flight members may result in that fighter having insufficient fuel to complete its subsequent mission, and thus must return to base earlier, reducing the combat effectiveness of the flight.
To mitigate this impact, combat refueling operations will usually require the entire flight to cycle through the tanker again a second time to top up each fighter's tanks. While this ensures that all fighters in the flight have as close to the same fuel state as far as possible, it also means the entire flight will be delayed departing the tanker to perform its mission.
The post-strike refueling situation may be even more critical: fighters coming off-target are likely to be low on fuel, and some may potentially be in “emergency fuel” states, or on the verge of engine flameout. In such situations, the receiver aircraft must expeditiously connect up with the tanker, take on just enough fuel to keep flying for a few more minutes then move aside for the next receiver aircraft; if the first few receiver aircraft took longer than was necessary, those at the end of the queue would have run out of fuel. In these scenarios, the limitation of single-boom tankers and its impact on refueling time becomes pronounced.
A possible way to reduce the overall refueling time is to have more booms available on station, i.e. make more tankers available to reduce the receiver-to-tanker ratio. The refueling operation then becomes determined by the fuel transfer rate, rather than the fuel quantity needs of the receiver aircraft; in other words more tankers are needed than is actually necessary to transfer the required quantity of fuel. This result in less than optimal use of the tankers: it increases the demand on the tanker fleet, and may not be possible when there are a limited number of tankers in an operator's fleet, or in wartime when concurrent operations will place competing demands on the tanker fleet.
The probe/drogue method, on the other hand, cannot be used for refueling receptacle-equipped receiver aircraft, unless the receiver aircraft is modified and/or fitted with a probe. Numerous types of receiver aircraft (e.g. the A-10, F-15, F-16, F-22) are equipped with the boom receptacle, and it would be impractical from a cost or technical perspective to modify these aircraft to incorporate a refueling probe. Typically, the type of refueling system, either the receptacle or probe, installed in a receiver aircraft must be designed in from the start. For cost, weight and technical complexity reasons, it is very rare for a receiver aircraft to have both the receptacle and probe simultaneously, as one system would be unused during a refueling, and therefore constitute “dead weight.”
A need therefore exists to provide a solution that seeks to address at least some of the above problems.